1. Field of the Invention
The present invention relates to an interface circuit, and more particularly, it relates to an interface circuit provided with means for correcting a resistance value of a terminator.
2. Description of the Related Art
Universal Serial Bus (USB) 2.0 interface circuit has been available as an interface circuit designed to provide high-speed serial data transfer. The USB 2.0 interface circuit has a high-speed transfer rate and, to suppress signal reflection, the impedance matching of a transmission system is important. For impedance matching, a terminator having a predetermined resistance value is provided. However, changes occur in the resistance value of the terminator because of changes in an ambient temperature, and other factors, causing impedance mismatching. Accordingly, the conventional interface circuits included the following types: an interface circuit provided with means for correcting the resistance value of the terminator at the time of resetting; an interface circuit provided with means for correcting the resistance value of the terminator only immediately after reset releasing; and an interface circuit provided with means for correcting the resistance value of the terminator at every constant periods after reset releasing.
However, in the case of the types respectively provided with the means for correcting the resistance value of the terminator at the time of resetting, and the means for correcting the resistance value of the terminator only immediately after the reset releasing, characteristic impedance was not satisfied when a long-time continuous operation caused a change in the resistance value of the terminator. In addition, in the case of the type provided with the means for correcting the resistance value at every constant periods after the reset releasing, the resistance value was corrected even during the transmitting/receiving of data, causing distortion in an output waveform at the time of the data transmission, and a reception error at the time of the data receiving.